Single-chamber kiln with combustion gas circulation

ABSTRACT

A single chamber kiln for firing ceramic bodies, such as molded carbon bodies and the like, with combustion gas, includes a kiln having side walls and defining a chamber for receiving the ceramic bodies. Hydrocarbon fuel burners are arranged in the walls in a first plane for supplying the combustion gas to the chamber and gas outlets are disposed in the walls and directed into the chamber. Substantially vertical passages join the outlets and blowers are arranged in the walls at a second plane and communicate with these passages for moving combustion gas therethrough to circulate the same through the chamber.

w v s i United States Patent [1 1 1 3 Krolie et a1. July it). 31973 [54]SINGLE-CHAMBER KILN WITH 2,191,438 2/l940 Breeler 263/28 COMBUSTION GASCiRCULATlON 2,849,221 8/1958 Cone et a1. 263/43 {75] Inventors: WiliriedKrohe, Niederhochstadt; FOREIGN PATENTS OR APPLICATIONS Berthold Riisch,812,271 4/1959 Great Britain 263/28 Frankfurt-Schwanheim, both ofGermany Primary Examiner.lohn J. Camby Attorney-Curt M. Avery, Arthur E.Wilfond, Herbert {73] Assignee. SlgrrElektrographit GmbH, L. Lerner andD aniel l Tick Meitmgen near Augsburg, Germany [22] Filed: May 26, 1971[57] ABSTRA CT [21] App! NO; 146,890 A single chamber kiln for firingceramic bodies, such as molded carbon bodies and the like, withcombustion i gas, includes a kiln having side walls and defining a g ppy Data chamber for receiving the ceramic bodies. Hydrocar- June 18, 1971Germany P 20 29 840.2 bon fuel burners are arranged in the walls in afirst plane for supplying the combustion gas to the chamber [52] US. Cl.432/176, 432/6, 432/145, 432/146 and gas outlets are disposed in thewalls and directed [51] Int. Cl. F27b 3/02 into the chamber.Substantially vertical passages join [58] Field of Search 263/28, 43 theoutlets and blowers are arranged in the walls at a second plane andcommunicate with these passages for [56] References Cited movingcombustion gas therethrough to circulate the UNITED STATES PATENTS samethrough the chamber.

3,172,647 3/1965 Remmey 263/28 8 Claims, 5 Drawing Figures =0=o o=o .1a5

2 Sheetwlihcet 1 Patented July 10, 1973 FIG] FIG. 2

CIIIIZZI Patented July 10, 1973 3,744,966

' FIGS a I m? ,1/

SlNGLE-fiHAMBER KlLN WITH COMBUSTION GAS CIRCULATION Our inventionrelates to a single-chamber kiln with combustion gas circulation forfiring ceramic bodies, particularly molded carbon bodies. The kiln isequipped with gas or oil burners, blowers for circulating combustiongases and ceramic or metallic firing containers.

in general, ceramic materials can be shaped only in the plasticcondition which is adjusted by the addition of plasticizers or bysuitable preconditioning. The bodies produced by pressing, casting orother forming methods are soft and have little stability and in order toattain sufficient strength must be heated to an elevated temperature toevaporate or dissociate the plasticizer. The strength is moreoverincreased by temperature-induced phase changes and sintering processes.

Starting materials for molded carbon bodies are ground coke, graphite orcarbon black, which are mixed with a binder that serves at the same timeas plasticizer, such as coal-tar pitch, tar or synthetic resin, and ifdesired additions of oils which reduce the forming resistance. Theplastic material is formed in extrusion, screw-type injection or diepresses or also by shaking or tamping and is subsequently heated totemperatures of up to about 1,200C in order to convert the binder intosolid coke. in this process, substantial amounts of gas andcorrespondingly high pressures are generated in the molded bodiesthrough the dissociation of volatile substances from the bindermaterials. If the internal pressure exceeds the strength of the body,fissures are formed through which the gas is directed to the outside.The destruction of molded bodies can be avoided if equilibrium existsbelow a critical pressure limit between the gas quantity newly generatedper unit of time and the gas quantity led off by difiusion.

The coke produced in the heating of the molded bodies also is not stablebut is subject to ordering processes connected with a reduction ofvolume. These ordering processes induce tensile stresses, the magnitudeof which is proportional to the maximum temperature difference in thebody. These stresses are also a frequent cause for smaller or largercracks which can preclude economic use of the molded bodies.

Firing of carbon bodies without rejects is possible only if criticalheating speeds which depend on quality, shape and critical temperaturedifferences within the bodies are not exceeded. These conditions areonly approximately met in the ring chamber kilns which are preferablyused for the firing of molded carbon bodies. Because of the manner inwhich the flame is directed, substantial temperature differences occurwithin a con tainer charge, and furthermore, temperature jumps cannot beavoided in charging the firing chamber.

it has therefore been proposed to fire molded bodies which are sensitiveto tiring and especially those of large size, in singlechamber kilnswith combustion gas circulation, the temperature of which can becontrolled independently of preceding or following chambers. The knownsingle-chamber kilns with combustion gas circulation are heated byburners arranged in the longitudinal or end surfaces of the kiln chamberand the combustion gases are circulated by blowers in the kiln ceilingwith a mutual spacing of about 3 meters, or by blowers arranged in thelateral oven walls. Firing containers which contain the molded bodies tobe fired are placed in the kiln chamber.

With the commonly used, mean heating-up speed of about 60 to 100C perday, non-uniform combustion gas velocity in the kiln chamber resultsfrom the known arrangement of burners and blowers, especially betweenthe firing containers and correspondingly different heat transfercoefficients.

Temperature differences in the vertical direction of about 20 to 30C andof about 10 to 30C in the horizontal direction are generated thereby inthe firing containers, and for larger shapes the firing reject rate is 3to 10 percent. A further disadvantage of the known kilns is that withthe blowers arranged in the kiln ceiling, the space below the blowerscannot be filled with firing containers, so that only to percent of thefloor area can be utilized.

Finally, the failure of one of the available two or three blowers willintroduce a symmetrical temperature distribution in the kiln chamber andlead to a higher firing reject rate.

Accordingly, it is an object of our invention to provide asingle-chamber kiln that avoids the disadvantages of the knownsingle-chamber kiln with combustion gas circulation.

It is another object of our invention to provide a single-chamber kilnwith combustion gas circulation for firing firing-sensitive moldedbodies without rejects. Subsidiary to this object it is an object of ourinvention to provide a single-chamber kiln with combustion gascirculation wherein the commonly used heating-up speeds and a smallvertical temperature difference in the firing containers during theentire travel through kiln are not exceeded.

According to a feature of the invention, the foregoing objects arerealized by arranging in the longitudinal walls of the kiln chamber,burners and injection holes in one plane, and in another plane, blowersat a mutual spacing of at most 1.5 meters. In addition, verticalcombustion gas shafts are provided between the blowers and the injectionholes.

According to another feature of the invention, the distance between theplane of the burners and the plane of the intake openings of thecirculation blowers is greater than the height of the firing containers,the plane of the burners being above the upper boundary plane of thefiring containers and the plane of the intake openings of thecirculation blowers below the lower boundary plane of the firingcontainers. According to the invention it is advantageous to arrangefiring containers of circular cross section in a quadrilateral array inthe kiln chamber, so that approximately 25 percent of the floor area isavailable for the circulation of the combustion gases and that thecombustion gases are conducted through geometrically identical channels.

According to a further feature of the invention, it is advantageous toconnect the vertical combustion gas shafts which start at the blowerswith gas collecting ducts or plenums built into the kiln ceiling. For abetter distribution of the combustion gas flow to the injection openingsand the plenum suitable slide dampers can be arranged in the plenum.

The advantages achieved with the single-chamber kiln with combustion gascirculation according to the invention consist particularly in thefeature that in firing of large-size molded bodies of qualitiessensitive to firing no fissures or other faults occur at the usualheating-up and cooling speed which might degrade the properties of themolded bodies so that rejects which up to now have been unavoidable canbe practically eliminated. Further advantages of the kiln according tothe invention are high thermal efficiency and the low sensitivityagainst the failure of individual circulation blowers and/or individualburners.

Single-chamber kilns with combustion gas circulation are made equippedwith a fixed floor and removable, integral or multisectional kilnceiling, and with a fixed ceiling and movable floor.

The invention will now be described with reference to the drawingwherein:

FIG. 1 is a lateral view, partially in section, of a single-chamber kilnaccording to the invention depicted as a car kiln in which firingcontainers are arranged on a car which is guided by tracks built intothe kiln floor;

FIG. 2 is a plan view of the single-chamber kiln of FIG. 1;

FIG. 3 is a cross section of the kilns of FIG. 1

FIG. 4, a detail from FIG. 3; and,

FIG. 5, a detail from FIG. 1.

Reference numeral 1 designates the kiln chamber which is equipped at theend face with the vertical-lift door 2. A car 3 is guided by tracks 4 onwhich containers 5 of sheet steel are arranged in a quadrilateralpattern. The car floor consists of refractory ceramic material and isloaded with the containers 5. The car floor has a multiplicity of slots6 (FIG. 5) arranged transversely to the longitudinal. axis of the car.In the side wall 7 of the kiln chamber 1 are arranged oil-fired burners8 and blowers 9 in two planes respectively. The burners and blowers aremutually spaced at 1.3 meters center-to-center respectively. If desired,the blowers and burners can be respectively mutually spaced up to 1.5meters center-to-center. This is the largest spacing permissible since agreater spacing would result in too great a vertical temperaturedifference in the firing container. The diameter of a blower is about0.5 meter.

In the burner plane between each two burners are provided injectionopenings 10 for the circulated gas. The injection openings 10 areconnected with the blowers 9 by vertical combustion gas shafts 11 whichopen into a plenum 12 in the kiln ceiling 13. The plenum l2 ends in theshaft 14 which has a control gate 15 and which leads to the stack 17 viathe post-combustion space 16.

After the car loaded with containers is run intothe kiln and thevertical-lift door is closed, the burners are ignited, from whichcombustion gas of temperature Tl flows past the containers 5 through theslots 6 in the car door to the blowers 9. The vertically descending flowof the hot circulating gases which due to the pattern arrangement of thecontainers is divided into many channels and which transfer a part oftheir heat content to the containers, assures automatically ahomogeneous temperature distribution in the planes parallel to theburner plane. The drawn-off combustion gas is then pushed by the blowers9 into the combustion gas shafts I l and from there for the most partinto the kiln chamber I via the injection openings 10. By admixing hotcombustion gas, the temperature of the circulating gas is increased tothe desired temperature T2 according to the heating program. By changingthe ratio of the circu lated gas quantity to the newly suppliedcombustion gas the vertical temperature difference in the kiln chambercan be controlled and in particular, can be made as small as desired.

A gas quantity corresponding to the newly supplied combustion gas isdrawn off by the plenum 12 and gets to the stack 17 via thepost-combustion space 16. Postcombustion is provided if the combustiongases contain olfactory or other detrimental impurities.

In the firing of cylindrical carbon molded bodies with a diameter of 0.6m and a height of 2.5 m in a car kiln according to the invention maximumvertical temperature differences of 10 to 15C and horizontal temperaturedifferences of 5 to 8C were measured. The firing reject rate was 1 to 3percent.

The single-chamber kiln according to the invention does not permit thecommonly used heating-up speeds to be exceeded. The single chamber kilnof the invention also does not permit the small vertical temperaturedifference in the firing containers to be exceeded during the entiretravel through the kiln which is necessary to obtain a firing of firingsensitive molded bodies with out rejects.

While the invention has been described by means of a specific exampleand in a specific embodiment, we do not wish to be be limited theretofor obvious modifications will occur to those skilled in the art withoutdeparting from the spirit and scope of the invention.

We claim:

1. A single-chamber kiln for firing ceramic bodies such as molded carbonbodies and the like, comprising .a kiln having side walls and definingachamber for receiving the ceramic bodies, a plurality of spaced burnermeans arranged in said walls in a first generally horizontal plane, aplurality of gas injection ports disposed in said walls, each of saidports arranged between adjacent bumer means, a plurality of suctionports disposed in said walls at a second generally horizontal planedisposed below said first plane, fans arranged in each of said suctionports, and substantially vertical passages disposed in said side wallsconnecting each of said suction ports with one injection port, wherebysaid fans pass the combustion gas from said suction ports through saidvertical passages to said injection ports to circulate the combustiongas through said chamber.

2. A single-chamber kiln according to claim 1, wherein said secondgenerally horizontal plane is disposed below said ceramic bodies andsaid first generally horizontal plane is disposed above said ceramicbody.

3. A single-chamber kiln according to claim 1 for firing ceramicmaterial contained in ceramic or steel containers placed in saidchamber, said burner means comprising a plurality of burners mutuallyspaced at intervals of at most 1.5 meters center-to-center and saidblower means comprising a plurality of blowers mutually spaced atintervals of at most 1.5 meters center-tocenter. I

4. In a single-chamber kiln according to claim 3, the distance betweensaid first plane and said second plane being greater than the height ofsaid containers.

5. In a single-chamber kiln according to claim 3, said containerseachhaving a circular cross-section and being grouped together andplaced in said chamber so that the periphery of the group outlines aquadrilateral.

6. A single-chamber kiln according to claim 3 comprising a conveyancefor moving said containers into and out of said chamber.

7. A single-chamber kiln according to claim 1, comprising plenum meansarranged above said injection ports, said passages extending upwardlybeyond said injection ports and-communicating with said plenum means forremoving a portion of the circulating combustion gas.

8. A single-chamber kiln according to claim 1, comprising an exhaustduct connected with said plenum means, and a control gate disposed insaid duct.

1. A single-chamber kiln for firing ceramic bodies such as molded carbon bodies and the like, comprising a kiln having side walls and defining a chamber for receiving the ceramic bodies, a plurality of spaced burner means arranged in said walls in a first generally horizontal plane, a plurality of gas injection ports disposed in said walls, each of said ports arranged between adjacent burner means, a plurality of suction ports disposed in said walls at a second generally horizontal plane disposed below said first plane, fans arranged in each of said suction ports, and substantially vertical passages disposed in said side walls connecting each of said suction ports with one injection port, whereby said fans pass the combustion gas from said suction ports through said vertical passages to said injection ports to circulate the combustion gas through said chamber.
 2. A single-chamber kiln according to claim 1, wherein said second generally horizontal plane is disposed below said ceramic bodies and said first generally horizontal plane is disposed above said ceramic body.
 3. A single-chamber kiln according to claim 1 for firing ceramic material contained in ceramic or steel containers placed in said chamber, said burner means comprising a plurality of burners mutually spaced at intervals of at most 1.5 meters center-to-center and said blower means comprising a plurality of blowers mutually spaced at intervals of at most 1.5 meters center-to-center.
 4. In a single-chamber kiln according to claim 3, the distance between said first plane and said second plane being greater than the height of said containers.
 5. In a single-chamber kiln according to claim 3, said containers each having a circular cross-section and being grouped together and placed in said chamber so that the periphery of the group outlines a quadrilateral.
 6. A single-chamber kiln according to claim 3 comprising a conveyance for moving said containers into and out of said chamber.
 7. A single-chamber kiln according to claim 1, comprising plenum means arranged above said injection ports, said passages extending upwardly beyond said injection ports and communicating with said plenum means for removing a portion of the circulating combustion gas.
 8. A single-chamber kiln according to claim 1, comprising an exhaust duct connected with said plenum means, and a control gate disposed in said duct. 